1. Field of the Invention
The present disclosure relates to a method of determining a mask pattern and an exposure conditions, and to a storage medium, and a computer therefor.
2. Description of the Related Art
An exposure apparatus is an apparatus that transfers a pattern formed on a mask (original) through a projection optical system onto a photosensitive substrate (a wafer or the like that has a resist layer formed on its surface) in a lithography step included in the manufacturing process for a semiconductor device or the like. Recently the exposure apparatus is configured to execute optimization of an illumination shape (effective light source distribution), which is used as an exposure condition when illuminating the mask, or the mask pattern by use of a super-resolution technique such as deformation illumination, optical proximity correction (OPC) or the like in order to comply with the advances in device miniaturization. During optimization of the illumination shape, firstly, a target pattern for the device (layout pattern), an evaluation position for the image-forming characteristics, and an evaluation item at the evaluation positions (e.g. dimension, depth of focus (DOF), exposure margin, or the like) are set. Then, calculation is performed of the transfer pattern (pattern to be formed on the image plane of the projection optical system) that is associated with the change in the illumination shape to thereby obtain values for the evaluation items at the evaluation positions on the transfer pattern. The calculation of the transfer pattern and the computation of the evaluation values are repeated until the values satisfy the evaluation standard or until the number of times of changes to the illumination shape reaches a predetermined number of times. At this time, the illumination shape can be expressed numerically, and for example, in annular illumination with certain intensity, the illumination shape is expressed by a function that uses an inner σ and outer σas parameters (variables). These parameters are optimized using a Monte Carlo Method or the like. However, even if the pattern of the mask is the same, when the illumination shape is changed, the transfer pattern also changes. Therefore, when the illumination shape is changed, there is the possibility that the transfer pattern will be shifted from the target pattern. Thus, OPC is required in order to ensure correspondence between the transfer pattern and the target pattern. OPC is executed on each occasion the illumination shape changes or when the illumination shape is changed by certain amount. The method of sequentially correcting the illumination shape and the mask pattern in this manner is known as a conventional Source Mask Optimization (SMO) technique.
In this regard, it is becoming increasingly difficult to form a pattern with sufficient accuracy on a mask to keep pace with the rapid developments in miniaturization. This is due to the fact that the mask pattern is separately optimized from the illumination shape. As disclosed above, OPC is generally executed after determination (optimization) of the illumination shape as a result of the dependence on the illumination shape. Therefore, since the mask pattern changes as a result of the execution of OPC, it may be the case that an illumination shape that is determined prior to the execution of OPC will no longer be optimal. In this context, Japanese Patent Application Laid-Open No. 2011-95729 discloses an SMO technique for the simultaneous optimization of the illumination shape and the mask pattern when handling a library cell or the like that is a circuit element or a memory cell of a device. This technique enables determination of both the illumination shape (illumination shape parameters) and the mask pattern (pattern parameters) to thereby enable formation of the transfer pattern with sufficient accuracy on a substrate.
When applied to device patterns that have a relatively large number of figures such as cell blocks or peripheral circuits, the technique disclosed in Japanese Patent Application Laid-Open No. 2011-95729 requires time for convergence of the optimization due to the large number of pattern parameters that form part of the optimization variables. As a result, for example, although it may be proposed to cut off the optimization calculation at a predetermined number of times, in that case, high accuracy pattern parameters or illumination shape parameters cannot be obtained.